The invention relates to a frequency changing device.
It is known to double the frequency of the light generated by a laser by means of a nonlinear optical medium, which is a crystal, for example. For effective frequency doubling it is necessary for light emerging from the laser to be coupled into the nonlinear optical medium with high efficiency. For this purpose, it is known for example from H. K. Nguyen et al.: 107-mW Low-Noise Green-Light Emission by Frequency Doubling of a Reliable 1060-nm DFB Semiconductor Laser Diode, IEEE Photonics Technologies Letters, Vol. 18, no. 5, Mar. 1, 2006 to use a lens system for coupling the laser and the crystal, which lens system images the light coupled out from the laser onto a small region of the entrance facet of the crystal.
Y. Kitaoka et al.: Wavelength Stabilization of a Distributed Bragg Reflector Laser Diode by Use of Complementary Current Injection, Optics Letters, Vol. 28, No. 11, Jun. 1, 2003, describes to couple the light from a laser embodied as a ridge waveguide laser directly, i.e. without a lens system, into a frequency-doubling crystal with integrated waveguide. A corresponding device is also described in W. P. Risk et al.: Compact Blue-Green Lasers, Cambridge University Press, 2003, chapters 6.3 and 6.3.1, which describe, inter alia, a butt coupling between a ridge waveguide laser diode and a frequency-doubling crystal with integrated waveguide.
The disadvantage of a butt coupling consists in undesired reflections at the exit facet of the laser and/or the entrance facet of the waveguide of the frequency-doubling crystal, which can lead to amplitude and/or frequency instabilities.